Although Calpurnia is well-educated--she taught Scout how to write cursive--and speaks very proper English in the Finch household, Scout notices that her language changes when she's at church among her fellow African-Americans. Scout suddenly realized that Calpurnia led "a modest double life," existing as the Finch housekeeper but going home each night into her own black world.
"Cal," I asked, "why do you talk nigger-talk to the--to your folks when you know it's not right?"
Cal explained that the way she spoke in the Finch house would be out of place among her black friends, just as it would if Jem and Scout "talked colored-folks' talk at home." She said it would be unladylike because it would sound to others like she knew more than they did.
"They'd think I was puttin' on airs to beat Moses."